1. Field of the Invention
The present invention relates to a direct file access system for a magnetic tape and to a system for reducing the number of tape retensioning passes. The present invention also relates to a reduced rewind data configuration on a magnetic tape, which may be used in combination with the direct file access system and the system for reducing the number of tape retensioning passes.
2. Description of the Related Art
Conventional tape organization is sequential, with records/files recorded serially along the length of the tape. Files typically are recorded on a single track from the Beginning of Tape (BOT) to the End of Tape (EOT). If the file is longer than the length of the tape, the tape drive steps the head to a different track and the recording process continues back from the EOT toward the beginning. Consequently, the multiple tracks on the tape are accessed in a serpentine manner so as to appear as one long tape to the tape drive.
Advanced tape organization systems place a directory of a few kilobytes at the BOT which allows the read/write head to be moved laterally across the tape to the desired track. The tape then is advanced in a high-speed ballistic manner to the approximate location of the beginning of the desired record.
However, even with optimum organization and search algorithms such as Quick File Access (QFA), the average access to an individual record may be anywhere from ten seconds to several minutes. A sequential search can take anywhere from ten minutes to hours.
Access time is becoming an even greater problem as tapes get longer. For example, based on 1/3 of the length of the tape access, a 1,200 feet (365 m) tape has an average access time of 40 seconds at 120 inches per second (ips) (3.0 m/s). It is projected that the tape length in new 5.25 form factor tape cartridges with one-quarter inch (6.3 mm) tape will increase to between 1,500 and 1,800 feet (460 and 550 m) in the near future, further increasing access time.
Another factor that slows access to data is the automatic refresh or retensioning procedures employed every time the cartridge is loaded into the drive. This procedure causes the entire tape to be wound and rewound, At worst, this results in a 2-4 minute delay for long tapes. Additionally, if a tape is shuffled repeatedly in a short region, or if its temperature varies greatly since it was last used, the tape can lose tension to a degree that will degrade data integrity, requiring additional time consuming retensioning procedures.
In order to take full advantage of the huge data capacity available on tape cartridges, a direct file access system, a system for reducing the number of retensioning passes, and a reduced rewind data configuration for magnetic tapes is needed to reduce the average access time, preferably to less than 2 seconds.